Packaged strain actuator

ABSTRACT

A modular actuator assembly includes one or more plates or elements of electro-active material bonded to an electroded sheet, preferably by a structural polymer to form a card. The card is sealed, and may itself constitute a practical device, such as a vane, shaker, stirrer, lever, pusher or sonicator for direct contact with a solid or immersion in a fluid, or may be bonded by a stiff adhesive to make a surface-to-surface mechanical coupling with a solid workpiece, device, substrate, machine or sample. The structural polymer provides a bending stiffness such that the thin plate does not deform to its breaking point, and a mechanical stiffness such that shear forces are efficiently coupled from the plate to the workpiece. In further embodiments, the card may include active circuit elements for switching, powering or processing signals, and/or passive circuit elements for filtering, matching or damping signals, so that few or no connections to outside circuitry are required. The actuator assembly can be manufactured in quantity, to provide a versatile actuator with uniform mechanical and actuation characteristics, that introduces negligible mass loading to the workpiece. The cards themselves may be arranged as independent mechanical actuators, rather than strain-transfer actuators, in which the induced strain changes the position of the card. Various arrangements of pinned or cantilevered cards may act as a pusher, bender or other motive actuator, and structures such as powered bellows may be formed directly by folding one or more suitably patterned cards.

This application is a continuation application of Ser. No. 08/188,145filed on Jan. 27, 1994. The contents of the aforementioned applicationare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to actuator elements such as may be usedfor active vibration reduction, structural control, dynamic testing,precision positioning, motion control, stirring, shaking, and passive oractive damping. More particularly, the present invention relates to apackaged actuator assembly that is electronically controllable and maybe used separately or adapted to actively suppress vibration, actuatestructures, or damp mechanical states of a device to which it isattached. As described in a subsequent section below, the assembly maybe bonded or attached to a structure or system, thereby integrating itwith the system to be actuated, controlled or damped.

Smart materials, such as piezoelectric, electrostrictive ormagnetostrictive materials, may be used for high band width tasks suchas actuation or damping of structural or acoustic noise, and also forprecision positioning applications. Such applications frequently requirethat the smart material be bonded or attached to the structure that itis to control. However, general purpose actuators of these materials arenot generally available, and typically a person wishing to implementsuch a control task must take raw, possibly non-electroded, smartmaterial stock, together with any necessary electrodes, adhesives andinsulating structures and proceed to fasten it onto, or incorporate itinto, the article of interest.

For such applications, it becomes necessary to connect and attach thesematerials in such a way that the mechanical and electrical connectionsto the smart material are robust and capable of creating strain withinthe smart member or displacing or forcing the system, and to couple thisstrain, motion or force to the object which is to be controlled. Often,it is required that the smart material be used in a non-benignenvironment, greatly increasing the chances of its mechanical orelectrical failure.

By way of example, one such application, that of vibration suppressionand actuation for a structure, requires attachment of a piezoelectricelement (or multiple elements) to the structure. These elements are thenactuated, the piezoelectric effect transforming electrical energyapplied to the elements into mechanical energy that is distributedthroughout the elements. By selectively creating mechanical impulses orchanging strain within the piezoelectric material, specific shapecontrol of the underlying structure is achievable. Rapid actuation canbe used to suppress a natural vibration or to apply a controlledvibration or displacement. Examples of this application of piezoelectricand other intelligent materials have become increasingly common inrecent years.

In a typical vibration suppression and actuation application, apiezoelectric element is bonded to a structure in a complex sequence ofsteps. The surface of the structure is first machined so that one ormore channels are created to carry electrical leads needed to connect tothe piezoelectric element. Alternatively, instead of machining channels,two different epoxies may be used to make both the mechanical and theelectrical contacts. In this alternative approach, a conductive epoxy isspotted, i.e., applied locally to form conductors, and a structuralepoxy is applied to the rest of the structure, bonding the piezoelectricelement to the structure. Everything is then covered with a protectivecoating.

This assembly procedure is labor intensive, and often involves muchrework due to problems in working with the epoxy. Mechanical uniformitybetween different piezoelectric elements is difficult to obtain due tothe variability of the process, especially with regard to alignment andbonding of the piezoelectric elements. Electrical and mechanicalconnections formed in this way are often unreliable. It is common forthe conductive epoxy to flow in an undesirable way, causing a shortacross the ends of the piezoelectric element. Furthermore, piezoelectricelements are very fragile and when unsupported may be broken duringbonding or handling.

Another drawback of the conventional fabrication process is that afterthe piezoelectric element is bonded to the structure, if fractureoccurs, that part of the piezoelectric element which is not in contactwith the conductor is disabled. Full actuation of the element is therebydegraded. Shielding also can be a problem since other Circuit componentsas well as personnel must generally be shielded from the electrodes ofthese devices, which may carry a high voltage.

One approach to incorporating piezoelectric elements, such as a thinpiezoelectric plate, a cylinder or a stack of discs or annuli, into acontrollable structure has been described in U.S. Pat. No. 4,849,668 ofJavier de Luis and Edward F. Crawley. This technique involves meticuloushand-assembly of various elements into an integral structure in whichthe piezoceramic elements are insulated and contained within thestructure of a laminated composite body which serves as a strongsupport. The support reduces problems of electrode cracking, and, atleast as set forth in that patent, may be implemented in a waycalculated to optimize structural strength with mechanical actuationefficiency. Furthermore, for cylinders or stacked annuli the naturalinternal passage of these off-the-shelf piezo forms simplifies, to someextent, the otherwise difficult task of installing wiring. Nonetheless,design is not simple, and fabrication remains time-consuming and subjectto numerous failure modes during assembly and operation.

The field of dynamic testing requires versatile actuators to shake orperturb structures so that their response can be measured or controlled.Here, however, the accepted methodology for shaking test devicesinvolves using an electro-mechanical motor to create a lineardisturbance. The motor is generally applied via a stinger design, inorder to decouple the motor from the desired signal. Such externalmotors still have the drawback that dynamic coupling is oftenencountered when using the motor to excite the structure. Furthermore,with this type of actuator, inertia is added to the structure, resultingin undesirable dynamics. The structure can become grounded when theexciter is not an integral part of the structure. These factors cangreatly complicate device behavior, as well as the modeling ormathematical analysis of the states of interest. The use ofpiezoelectric actuators could overcome many of these drawbacks, but, asnoted above, would introduce its own problem of complex construction,variation in actuation characteristics, and durability. Similar problemsarise when a piezoelectric or electrostrictive element is used forsensing.

Thus, improvements are desirable in the manner in which an element isbonded to the structure to be controlled or actuated, such that theelement may have high band width actuation capabilities and be easilyset up, yet be mechanically and electrically robust, and notsignificantly alter the mechanical properties of the structure as awhole. It is also desirable to achieve high strain transfer from thepiezoelectric element to the structure of interest.

SUMMARY OF THE INVENTION

An actuator assembly according to the present invention includes one ormore strain elements, such as a piezoelectric or electrostrictive plateor shell, a housing forming a protective body about the element, andelectrical contacts mounted in the housing and connecting to the strainelement, these parts together forming a flexible card. At least one sideof the assembly includes a thin sheet which is attached to a major faceof the strain element, and by bonding the outside of the sheet to anobject a stiff shear-free coupling is obtained between the object andthe strain element in the housing.

In a preferred embodiment, the strain elements are piezoceramic plates,which are quite thin, preferably between slightly under an eighth of amillimeter to several millimeters thick, and which have a relativelylarge surface area, with one or both of their width and lengthdimensions being tens or hundreds of times greater than the thicknessdimension. A metallized film makes electrode contact, while a structuralepoxy and insulating material hermetically seal the device againstdelamination, cracking and environmental exposure. In a preferredembodiment, the metallized film and insulating material are bothprovided in a flexible circuit of tough polymer material, which thusprovides robust mechanical and electrical coupling to the enclosedelements.

By way of illustration, an example below describes a constructionutilizing rectangular PZT plates a quarter millimeter thick, with lengthand width dimensions each of one to three centimeters, each element thushaving an active strain-generating face one to ten square centimeters inarea. The PZT plates are mounted on or between sheets of a stiff strongpolymer, e.g., one half, one or two mil polymide, which is copper cladon one or both sides and has a suitable conductive electrode patternformed in the copper layer for contacting the PZT plates. Variousspacers surround the plates, and the entire structure is bonded togetherwith a structural polymer into a waterproof, insulated closed package,having a thickness about the same as the plate thickness, e.g., 0.30 to0.50 millimeters. So enclosed, the package may bend; extend and flex,and undergo sharp impacts, without fracturing the fragile PZT elementswhich are contained within. Further, because the conductor pattern isfirmly attached to the polymide sheet, even cracking of the PZT elementdoes not sever the electrodes, or prevent actuation over the full areaof the element, or otherwise significantly degrade its performance.

The thin package, forms a complete modular unit, in the form of a small"card", complete with electrodes. The package may then conveniently beattached by bonding one face to a structure so that it couples strainbetween the enclosed strain element and the structure. This may be donefor example, by simply attaching the package with an adhesive toestablish a thin, high shear strength, coupling with the PZT plates,while adding minimal mass to the system as a whole. The plates may beactuators, which couple energy into the attached structure, or sensorswhich respond to strain coupled from the attached structure.

In different embodiments, particular electrode patterns are selectivelyformed on the sheet to either pole the PZT plates in-plane orcross-plane, and multiple layers of PZT elements may be arranged orstacked in a single card to result in bending or shear, and evenspecialized torsional actuation.

In accordance with a further aspect of the invention, circuit elementsare formed in, or with, the modular package to filter, shunt, or processthe signal produced by the PZT elements, to sense the mechanicalenvironment, or even to locally perform switching or power amplificationfor driving the actuation elements. The actuator package may be formedwith pre-shaped PZT elements, such as half-cylinders, into modularsurface-mount shells suitable for attaching about a pipe, rod or shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other desirable properties of the invention will be understoodfrom the detailed description of illustrative embodiments, wherein:

FIG. 1A is a system illustration of a typical prior art actuator;

FIGS. 1B and 1C are corresponding illustrations of two systems inaccordance with the present invention;

FIGS. 2A and 2B show top and cross-sectional views, respectively, of abasic actuator or sensor card in accordance with the present invention;

FIG. 2C illustrates an actuator or sensor card with circuit elements;

FIG. 3 illustrates another card;

FIGS. 4A and 4B show sections through the card of FIG. 3;

FIGS. 5 and 5A show details of the layer structure of the card of FIG.3;

FIG. 6 shows an actuator package comb electrodes for in-plane actuation;

FIG. 7 illustrates a torsional actuator package using the cards of FIG.6;

FIGS. 8A and 8B show actuators mounted as surface mount actuators on asurface or rod, respectively; and

FIG. 9 shows actuators mounted as mechanical elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates in schema the process and overall arrangement of aprior art surface mounted piezoelectric actuator assembly 10. Astructure 20, which may be a structural or machine element, a plate,airfoil or other interactive sheet, or a device or part thereof has asheet 12 of smart material bonded thereto by some combination ofconductive and structural polymers, 14, 16. An insulator 18, which maybe formed entirely or in part of the structural polymer 16, encloses andprotects the smart material, while conductive leads or surfaceelectrodes are formed or attached by the conductive polymer. An externalcontrol system 30 provides drive signals along lines 32a, 32b to thesmart material, and may receive measurement signals from surface-mountedinstrumentation such as a strain gauge 35, from which it derivesappropriate drive signals. Various forms of control are possible. Forexample, the strain gauge may be positioned to sense the excitation of anatural resonance, and the control system 30 may simply actuate the PZTelement in response to a sensor output, so as to stiffen the structure,and thereby shift its resonant frequency. Alternatively, a vibrationsensed by the sensor may be fed back as a processed phase-delayeddriving signal to null out an evolving dynamic state, or the actuatormay be driven for motion control. In better-understood mechanicalsystems, the controller may be programmed to recognize empiricalconditions, i.e., aerodynamic states or events, and to select specialcontrol laws that specify the gain and phase of a driving signal foreach actuator 12 to achieve a desired change.

For all such applications, major work is required to attach the bare PZTplate to its control circuitry and to the workpiece, and many of theassembly steps are subject to failure or, when quantitative control isdesired, may require extensive modeling of the device after it has beenassembled, in order to establish control parameters for a useful mode ofoperation that are appropriate for the specific thicknesses andmechanical stiffnesses achieved in the fabrication process.

FIG. 1B shows an actuator according to one embodiment of the presentinvention. As shown, it is a modular pack or card 40 that simplyattaches to a structure 20 with a quick setting adhesive, such as afive-minute epoxy 13, or in other configurations attaches at a point orline. The operations of sensing and control thus benefit from a morereadily installable and uniformly modeled actuator structure. Inparticular, the modular pack 40 has the form of a card, a stiff butbendable plate, with one or more electrical connectors preferably in theform of pads located at its edge (not shown) to plug into a multi-pinsocket so that it may connect to a simplified control system 50. Asdiscussed in greater detail below with respect to FIG. 2C, the modularpackage 40 may also incorporate planar or low-profile circuit elements,which may include signal processing elements, such as weighting orshunting resistors, impedance marchers, filters and signal conditioningpreamplifiers, and may further include switching transistors and otherelements to operate under direct digital control, so that the onlyexternal electrical connections necessary are those of a microprocessoror logic controller, and a power supply.

In a further embodiment particularly applicable to some low powercontrol situations, a modular package 60 as shown in FIG. 1C may includeits own power source, such as a battery or power cell, and may include acontroller, such as a microprocessor chip or programmable logic array,to operate on-board drivers and shunts, thus effecting a complete set ofsensing and control operations without any external circuit connections.

The present invention specifically pertains to piezoelectric polymers,and to materials such as sintered metal zirconate, niobate crystal orsimilar piezoceramic materials that are stiff, yet happen to be quitebrittle. It also pertains to electrostrictive materials. As used in theclaims below, both piezoelectric and electrostrictive elements, in whichthe material of the elements has an electromechanical property, will bereferred to as electro-active elements. High stiffness is essential forefficiently transferring strain across the surface of the element to anoutside structure or workpiece, typically made of metal or a hardstructural polymer, and the invention in its actuator aspect does notgenerally contemplate soft polymer piezoelectric materials. While theterms "stiff" and "soft" are relative, it will be understood that inthis context, the stiffness, as applied to an actuator, is approximatelythat of a metal, cured epoxy, high-tech composite, or other stiffmaterial, with a Young's modulus greater than 0.1×10⁶, and preferablygreater than 0.2×10⁶. When constructing sensors, instead of actuators,the invention also contemplates the use of low-stiffness piezoelectricmaterials, such as polyvinylidene difluoride (PVDF) film and thesubstitution of lower cure temperature bonding or adhesive materials.The principal construction challenges, however, arise with the firstclass of piezo material noted above, and these will now be described.

In general, the invention includes novel forms of actuators and methodsof making such actuators, where "actuator" is understood to mean acomplete and mechanically useful device which, When powered, couplesforce, motion or the like to an object or structure. In its broad form,the making of an actuator involves "packaging" a raw electro-activeelement to make it mechanically useful. By way of example, rawelectro-active piezoelectric materials or "elements" are commonlyavailable in a variety of semi-processed bulk material forms, includingraw piezoelectric material in basic shapes, such as sheets, rings,washers, cylinders and plates, as well as more complex or compositeforms, such as stacks, or hybrid forms that include a bulk material witha mechanical element, such as a lever. These materials or raw elementsmay have metal coated on one or more surfaces to act as electricalcontacts, or may be non-metallized. In the discussion below,piezoelectric materials shall be discussed by way of example, and allthese forms of raw materials shall be referred to as "elements","materials", or "electro-active elements". As noted above, the inventionfurther includes structures or devices made by these methods andoperating as transducers to sense, rather than actuate, a strain,vibration, position or other physical characteristic, so that whereapplicable below, the term "actuator" may include sensing transducers.

Embodiments of the invention employ these stiff electrically-actuatedmaterials in thin sheets-discs, annuli, plates and cylinders orshells-that are below several millimeters in thickness, andillustratively about one fifth to one quarter millimeter thick.Advantageously, this thin dimension allows the achievement of highelectric field strengths across a distance comparable to the thicknessdimension of the plate at a relatively low overall potential difference,so that full scale piezoelectric actuation may be obtained with drivingvoltages often to fifty volts, or less. Such a thin dimension alsoallows the element to be attached to an object without greatly changingthe structural or physical response characteristics of the object.However, in the prior art, such thin elements are fragile, and may breakdue to irregular stresses when handled, assembled or cured. The impactfrom falling even a few centimeters may fracture a piezoceramic plate,and only extremely small bending deflections are tolerated beforebreaking.

In accordance with the present invention, the thin electrically actuatedelement is encased by layers of stiff insulating material, at least oneof which is a tough film which has patterned conductors on one of itssurfaces, and is thinner than the element itself. A package is assembledfrom the piezo elements, insulating layers, and various spacers orstructural fill material, such that altogether the electrodes, piezoelement(s), and enclosing films or layers form a sealed card of athickness not substantially greater than that of the bare actuatingelement. Where elements are placed in several layers, as will bedescribed below, the package thickness is not appreciably greater thanthe sum of the thicknesses of the stacked actuating elements.

FIG. 2A illustrates a basic embodiment 100 of the invention. A thin film110 of a highly insulating material, such as a polyimide material, ismetallized, typically copper clad, on at least one side, and forms arectangle which is coextensive with or slightly larger than the finishedactuator package. A suitable material available for use in fabricatingmultilayer circuit boards is distributed by the Rogers Corporation ofChandler, Arizona as their Flex-I-Mid 3000 adhesiveless circuitmaterial, and consists of a polyimide film formed on a rolled copperfoil. A range of sizes are available commercially, with the metal foilsbeing of 18 to 70 micrometer thickness, integrally coated with apolyimide film of 13 to 50 micrometer thickness. Other thicknesses maybe fabricated. In this commercial material, the foil and polymer filmare directly attached without adhesives, so the metal layer may bepatterned by conventional masking and etching, and multiple patternedlayers may be built up into a multilayer board in a manner describedmore fully below, without residual adhesive Weakening the assembly orcausing delamination. The rolled copper foil provides high in-planetensile strength, while the polyimide film presents a strong, tough anddefect-free electrically insulating barrier.

In constructions described below, the film constitutes not only aninsulator over the electrodes, but also an outer surface of the device.It is therefore required to have high dielectric strength, high shearstrength, water resistance and an ability to bond to other surfaces.High thermal resistance is necessary in view of the temperature cureused in the preferred fabrication process, and is also required for someapplication environments. In general, polyamide/imides have been founduseful, but other materials, such as polyesters with similar properties,may also be used.

In the present constructions, the foil layer is patterned byconventional masking and etch techniques (for example, photoresistmasking and patterning, followed by a ferric chloride etch), to formelectrodes for contacting the surface of piezo plate elements.Electrodes 111 extend over one or more sub-regions of the interior ofthe rectangle, and lead to reinforced pads or lands 111a, 111b extendingat the edge of the device. The electrodes are arranged in a pattern tocontact a piezoelectric element along a broadly-turning path, whichcrosses the full length and width of the element, and thus assures thatthe element remains connected despite the occurrence of a few cracks orlocal breaks in the electrode or the piezo element. Frame members 120are positioned about the perimeter of sheet 110, and at least onepiezoelectric plate element 112 is situated in the central region sothat it is contacted by the electrodes 111. The frame members serve asedge binding, so that the thin laminations do not extend to the edge,and they also function as thickness spacers for the hot-press assemblyoperation described further below, and as position-markers which definethe location of piezo plates that are inserted during the initial stagesof assembling the laminated package.

FIG. 2A is a somewhat schematic view, inasmuch as it does not show thelayer structure of the device which secures it together, including afurther semi-transparent top layer 116 (FIG. 2B), which in practiceextends over the plate 112 and together with the spacers 120 and sheet110 closes the assembly. A similar layer 114 is placed under the piezoelement, with suitable cut-outs to allow the electrodes 111 to contactthe element. Layers 114, 116 are preferably formed of a curable epoxysheet material, which has a cured thickness equal to the thickness ofthe metal electrode layer, and which acts as an adhesive layer to jointogether the material contacting it on each side. When cured, this epoxyconstitutes the structural body of the device, and stiffens theassembly, extending entirely over a substantial portion of the surfaceof the piezo element to strengthen the element and arrest crack growth,thereby enhancing its longevity. Furthermore, applicant has found thatepoxy from this layer actually spreads in a microscopically thin buthighly discontinuous fill, about 0.0025 mm thick, over the electrodes,bonding them firmly to the piezo plate, but with a sufficient number ofvoids and pinholes so that direct electrical contact between theelectrodes and piezo elements still occurs over a substantial anddistributed contact area.

FIG. 2B shows a cross-sectional view, not dram to scale, of theembodiment of FIG. 2A. By way of rough proportions, taking thepiezoelectric plate 112 as 0.2-0.25 millimeters in thickness, theinsulating film 110 is much thinner, no more than one-tenth to one-fifththe plate thickness, and the conductive copper electrode layer 111 mayhave a thickness typically of ten to fifty microns, although the latterrange is not a set of strict limits, but represents a useful range ofelectrode thicknesses that are electrically serviceable, convenient tomanufacture and not so thick as to either impair the efficiency ofstrain transfer or introduce delamination problems. The structural epoxy114 fills the spaces between electrodes 111 in each layer, and hasapproximately the same thickness as those electrodes, so that the entireassembly forms a solid bock. The spacers 120 are formed of a relativelycompressible material, having a low modulus of elasticity, such as arelatively uncrosslinked polymer, and, when used with a pressure-curedepoxy as described below, are preferably of a thickness roughlyequivalent to the piezoceramic plate or stack of elements, so that theyform art edge binding about the other components between the top andbottom layers of film 110.

A preferred method of manufacture involves applying pressure to theentire package as the layer 116 cures. The spacers 120 serve to alignthe piezoceramic plates and any circuit elements, as described belowwith reference to FIGS. 3-5, and they form a frame that is compressedslightly during assembly in the cure step, at which time it may deformto seal the edges without leaving any stress or irregularities.Compression eliminates voids and provides a dense and crack-free solidmedium, while the curing heat effects a high degree of cross-linking,resulting in high strength and stiffness.

An assembly process for the embodiment of FIGS. 2A, 2B is as follows.One or more pieces of copper clad polyimide film, each approximately0.025 to 0.050 millimeters thick in total, are cut to a size slightlylarger than the ultimate actuator package dimensions. The copper side ofthe film is masked and patterned to form the desired shape of electrodesfor contacting a piezo element together with conductive leads and anydesired lands or access terminals. A pitchfork electrode pattern isshown, having three tines which are positioned to contact the center andboth sides of one face of a piezo element, but in other embodiments anH- or a comb-shape is used. The patterning may be done by masking,etching and then cleaning, as is familiar from circuit board orsemiconductor processing technology. The masking is effected byphotoresist patterning, screening, tape masking, or other suitableprocess. Each of these electroded pieces of polyimide film, like aclassical printed circuit board, defines the positions of circuitelements or actuator sheets, and will be referred to below simply as a"flex circuit."

Next, uncured sheet epoxy material having approximately the samethickness or slightly thicker than the electrode foil layer is cut,optionally with through-apertures matching the electrode pattern toallow enhanced electrical contact when assembled, and is placed overeach flex circuit, so it adheres to the flex circuit and forms aplanarizing layer between and around the electroded portions. Thebacking is then removed from the epoxy layers attached to the flexcircuits, and pre-cut spacers 120 are placed in position at comer andedges of the flex circuit. The spacers outline a frame which extendsabove the plane of the electrodes, and defines one or more recesses intowhich the piezo elements are to be fitted in subsequent assembly steps.The piezo element or elements are then placed in the recesses defined bythe spacers, and a second electroded film 111, 112 with its ownplanarizing/bonding layer 114 is placed over the element in a positionto form electrode contacts for the top of the piezo element. If thedevice is to have several layers of piezo elements, as would be the casefor some bending actuator constructions, these assembly steps arerepeated for each additional electroded film and piezoelectric plate,bearing in mind that a polymide film which is clad and patterned on bothsides may be used when forming an intermediate electrode layer that isto contact actuator elements both above and below the intermediatesheet.

Once all elements are in place, the completed sandwich assembly ofpatterned flex circuits, piezo sheets, spacers and curable patternedepoxy layers is placed in a press between heated platens, and is curedat an elevated temperature and pressure to harden the assembly into astiff, crack-free actuator card. In a representative embodiment, a curecycle of thirty minutes at 350° F. and 50-100 psi pressure is used. Theepoxy is selected to have a curing temperature below the depolingtemperature of the piezo elements, yet achieve a high degree ofstiffness.

The above construction illustrates a simple actuator card having asingle piezo plate sandwiched between two electroded films, so that theplate transfers shear strain efficiently through a thin film to thesurface of the actuator card. The measure of transfer efficiency, givenby the shear modulus divided by layer thickness squared, and referred toas gamma (Γ), depends on the muduli and thickness of the epoxy 114, therolled foil electrodes 111, and the polyimide film 110. In arepresentative embodiment in which the epoxy and copper electrode layersare 1.4 mils thick and the epoxy has a modulus of 0.5×10⁶, a gamma ofapproximately 9×10¹⁰ pounds/inch⁴ is achieved. Using a thinner epoxylayer and film with 0.8 mil foil, substantially higher Γ is achieved. Ingeneral, the gamma of the electrode/epoxy layer is greater than 5×10¹⁰pounds/inch⁴, while that of the film is greater than 2×10¹⁰pounds/inch⁴.

It should be noted that using PZT actuator plates ten mils thick, a cardhaving two PZT plates stacked over each other with three flex circuitelectroded film layers (the middle one being double clad to contact bothplates) has a total thickness of 28 mils, only forty percent greaterthan the plates alone. In terms of mass loading, the weight of theactuator elements represents 90% of the total weight of this assembly.Generally, the plates occupy fifty to seventy percent of the packagethickness, and constitute seventy to ninety percent of its mass, inother constructions. Thus, the actuator itself allows near-theoreticalperformance modeling. This construction offers a high degree ofversatility as well, for implementing benders (as just described) aswell as stacks or arrays of single sheets.

Another useful performance index of the actuator constructed inaccordance with the present invention is the high ratio of actuatorstrain ε to the free piezo element strain Λ, which is approximately(0.8) for the two layer embodiment described herein, and in general isgreater than (0.5). Similarly, the ratio of package to free elementcurvatures, K, is approximately 0.85-0.90 for the describedconstructions, and in general is greater than 0.7.

Thus, overall, the packaging involved in constructing a piezo elementembedded in a flex circuit impairs its weight and electromechanicaloperating characteristics by well under 50%, and as little as 10%, whileprofoundly enhancing its hardiness and mechanical operating range inother important respects. For example, while the addition of sheetpackaging structure to the base element would appear to decreaseattainable K, in practical use the flex card construction results inpiezo bender constructions wherein much greater total deflection may beachieved, since large plate structures may be fabricated and highcurvature may be repeatedly actuated, without crack failure or othermechanical failure modes arising. Several Figures will illustrate thevariety of constructions to which such enhanced physical characteristicsare brought.

First, applicant notes that the structure of an electro-active elementembedded between flex circuits not only provides a low mass unifiedmechanical structure with predictable response characteristics, but alsoallows the incorporation of circuit elements into or onto the actuatorcard. FIG. 2C shows a top view of one device 70 of this type, whereinregions 71, 73 each contain broad electro-active sheets, while a centralregion 72 contains circuit or power elements, including a battery 75, aplanar power amplification or set of amplifiers 77, a microprocessor 79,and a plurality of strain gauges 78. Other circuit elements 82a, 82b maybe located elsewhere along the path of circuit conductors 81 about theperiphery. As with the other embodiments, spacers 120 define layout andseal edges of the device, while electrodes 111 attach the electro-activeelements to the processing or control circuitry which is now built-in.The circuit elements 82a, 82b may comprise weighting resistors if thedevice is operated as a sensor, or shunting resistors to implementpassive damping control. Alternatively, they may be filtering,amplifying, impedance matching or storage elements, such as capacitors,amplifiers or the like. In any case, these elements also are locatedaway from electro-active plates 84. The components collectively maysense strain and implement various patterns of actuation in response tosensed conditions, or perform other sensing or control tasks.

Returning now to the actuator aspect of the invention, FIG. 3 shows atop view of an actuator package 200 having dimensions of about1.25×9.00×0.030 inches and assembled with two layers of piezoelectricplates of four plates each. A rectangular polyimide sheet 210 with anend tab 210a carries an electrode 211 in the form of a lattice ofH-shaped thin copper lines interconnected to each other and to a singlerunner 211a that leads out to the tab, thus providing a low impedanceconnection directly to each of four rectangular regions which hold thepiezo plates.

Spacer elements 220a, 220b of H-shape, or 220c of L-shape mark offcorners and delineate the rectangular spaces for location of the piezoplates 216. In this embodiment, a plurality of gaps 230, discussedfurther below, appear between adjacent the H- or L- spacers. As will beapparent from the description below, the use of these small discretespacer elements (I-, T- or O-shaped spacers may also be convenient) isenhanced because they may be readily placed on the tacky bonding epoxylayer 114 during assembly to mark out assembly positions and form areceiving recess for the piezo elements, However, the spacer structureis not limited to such a collection of discrete elements, but may be asingle or couple of frame pieces, formed as a punched-out sheet ormolded frame, to provide all, or one or more, orienting and/or sealingedges, or recesses for holding actuation of circuit components.

FIG. 5 illustrates a top view of each of the three sheet, electrode andpiezo plate layers separately, while FIG. 5A illustrates the generallayering sequence of the film, conductor, and spacer/piezo layers. Asshown., the spacers 220 and piezo plates 216 constitute a single layerbetween each pair of electrode layers.

FIGS. 4A and 4B (not dram to scale) illustrate the layer structure ofthe assembled actuator along the vertical sections at the positionsindicated by "A" and "B" in FIG. 3. As more clearly shown in FIG. 4A, apatterned bonding layer of epoxy sheet 214 is coplanar with eachelectrode layer 211 and fills the space between electrodes, while thespacer 220c is coplanar with the piezo plate 216 and substantially thesame thickness as the plate or slightly thicker. Illustratively, thepiezo plate 216 is a PZT-5A ceramic plate, available commercially in afive to twenty rail thickness, and has a continuous conductive layer216a covering each face for contacting the electrodes 211. The spacers220 are formed of somewhat compressible plastic with a softeningtemperature of about 250° C. This allows a fair degree of conformabilityat the cure temperature so the spacer material may fill slight voids214a (FIG. 4A) during the assembly process. As shown in FIG. 4B, thegaps 230 (when provided) between spacers result in openings 214b whichvent excess epoxy from the curable bonding layers 214, and fill withepoxy during the cure process. As illustrated in that FIGURE, a certainamount of epoxy also bleeds over into patches of film 215 between theelectrodes 211 and the piezo plate 216. Because of the large andcontinuous extent of electrode 211, this patchy leakage of epoxy doesnot impair the electrical contact with the piezo elements, and theadditional structural connection it provides helps prevent electrodedelamination.

It will be appreciated that with the illustrated arrangements ofelectrodes, each in vertically stacked pair of piezo plates may beactuated in opposition to each other to induce bending, or more numerousseparate electrodes may be provided to allow different pairs of platesto be actuated in different ways. In general, as noted above, theinvention contemplates even quite complex systems involving manyseparate elements actuated in different ways, with sensing, control, andpower or damping elements all mounted on the same card. In this regard,great flexibility in adapting the card to practical tasks is furtherprovided by its flexibility. In general, it has a supple flexibilitycomparable to that of an epoxy strip thirty mils thick, so that it maybe bent, struck or vibrated without damage. It may also be sharply bentor curved in the region of its center line CL (FIG. 3) where no piezoelements are encased, to conform to an attaching surface or corner. Theelements may be poled to change dimension in-plane or cross-plane, andthe actuators may therefore be attached to transmit strain to anadjacent surface in a manner effective to perform any of theabove-described control actions, or to launch particular waveforms ortypes of acoustic energy, such as flexural, shear or compressional wavesinto an adjacent surface.

FIG. 6 shows another actuator embodiment 300. In this embodiment,illustrated schematically, the epoxy bonding layer, film and spacerelements are not shown, but only electrode and piezo sheets areillustrated to convey the operative mechanisms. A first set ofelectrodes 340 and second set 342 are both provided in the same layer,each having the shape of a comb with the two combs interdigitated sothat an electrical actuation field is set up between the tooth of onecomb and an adjacent tooth of the other comb. A parallel pair of combs340a, 342a is provided on the other side of the piezo sheet, with combelectrode 340 tied to 340a, and comb electrode 342 tied to 342a, so asto set up an electric field with equipotential lines "e" extendingthrough the piezo sheet, and in-plane potential gradient between eachpair of teeth from different combs. The piezoceramic plates are notmetallized, so direct electrical contact is made between each comb andthe plate. The plates are poled in-plane, by initially applying a highvoltage across the combs to create a field strength above one twothousand volts per inch directed along the in-plane direction. Thisorients the piezo structure so that subsequent application of apotential difference across the two-comb electrodes results in in-plane(shear) actuation.

As shown in FIG. 7, two such actuators 300 may be crossed to providetorsional actuation.

In discussing the embodiments above, the direct transfer of strainenergy through the electrode/polyimide layer to any adjoining structurehas been identified as a distinct and novel advantage. Such operationmay be useful for actuation tasks or diverse as airfoil shape controlactuation and noise or vibration cancellation or control. FIGS. 8A and8B illustrates typical installations of flat (FIG. 8A) andhemicylindrical (FIG. 8B) embodiments 60 of the actuator, applied to aflat or slightly curved surface, and a shaft, respectively.

However, while the electromechanical materials of these actuatorsoperate by strain energy conversion, applications of the presentinvention extend beyond strain-coupling through the actuator surface,and include numerous specialized mechanical constructions in which themotion, torque or force applied by the actuator as a whole is utilized.In each of these embodiments, the basic strip- or shell-shaped sealedactuator is employed as a robust, springy mechanical element, pinned orconnected at one or more points along its length. As shown in FIG. 9,when electrically actuated, the strip then functions, alone or withother elements, as a self-moving lever, flap, leaf spring, stack orbellows. In the diagrams of FIGS. 9(a)-9(q), the elements A,A', A". . .are strip or sheet actuators such as shown in the above FIGURES, whilesmall triangles indicate fixed or pinned positions which correspond, forexample, to rigid mounting points or points of connection to astructure. Arrows indicate a direction of movement or actuation or thecontact point for such actuation, while L indicates a lever attached tothe actuator and S indicates a stack element or actuator.

The configurations of FIGS. 9(a)-9(c) as stacks, benders, or pinnedbenders may replace many conventional actuators. For example, acantilevered beam may carry a stylus to provide highly controlledsingle-axis displacement to constitute a highly linear, largedisplacement positioning mechanism of a pen plotter. Especiallyinteresting mechanical properties and actuation characteristics areexpected from multi-element configurations 9(d) et seq., whichcapitalize on the actuators having a sheet extent and being mechanicallyrobust. Thus, as shown in FIGS. 9(d) and (e), a pin-pin bellowsconfiguration may be useful for extended and precise one-axis Z-movementpositioning, by simple face-contacting movement, for applications suchas camera focusing; or may be useful for implementing a peristalsis-typepump by utilizing the movement of the entire face bearing against afluid. As noted in connection with FIG. 3, the flex circuit is highlycompliant, so hinged or folded edges may be implemented by simplyfolding along positions such as the centerline in FIG. 3, allowing aclosed bellows assembly to be made with small number of large,multi-element actuator units. The flex circuit construction allowsstrips or checkerboards of actuator elements to be laid out with foldlines between each adjacent pair of elements, and the fold lines may beimpressed with a thin profile by using a contoured (e.g. waffle-iron)press platen during the cure stage. With such a construction, an entireseamless bellows or other folded actuator may be made from a single flexcircuit assembly.

Applicant has further utilized such actuators to perform simplemechanical motions, such as bending, twisting or wiggling, applied toportions of a face mask or puppet for theatrical animation, and hasfound the actuators to have excellent actuation characteristics andversatile mounting possibilities for small-load, medium displacementactuation tasks of this type.

In general, tasks capable of implementation with pneumatic actuators ofsmall or medium displacement may be addressed using the flex circuitactuator cards of the invention as discrete mechanical elements, andwhere the task involves a structure such as a sheet, flap or wall, theflex circuit itself may constitute that structural component Thus, theinvention is suited to such functions as self-moving stirring vanes,bellows or pump walls, mirrors, and the like. In addition, as notedabove, tasks involving surface coupling of small displacement acousticor ultrasonic band frequency are also readily implemented with the lowmass highly coupled flex circuit actuators.

As noted above, the piezo element need not be a stiff ceramic element,and if the flex circuit is to be used only as a sensor, then either aceramic element, or a soft material such as PVDF may be employed. In thecase of the polymer, a thinner more pliant low temperature adhesive isused for coupling the element, rather than a hard curable epoxy bondinglayer.

The foregoing description of methods of manufacture and illustrativeembodiments is presented to indicate the range of constructions to whichthe invention applies. The invention having overcome numerous drawbacksin the fragility, circuit configuration and general utility of strainactuators, strain activated assemblies and sensors, other variations inthe physical architecture and practical applications of the modular flexcircuit actuators and sensors of the invention will occur to thoseskilled in the art, and such variations are considered to be within thescope of the invention in which patent fights are asserted, as set forthin the claims appended hereto.

What is claimed is:
 1. A piezoceramic package comprising a plurality of layers laminated about a sheet of piezoceramic material, and includingsaid sheet of piezoceramic material having a first surface and a second surface opposite to said first surface a flexible circuit, covering all of an active surface selected from among said first surface and said second surface of said piezoceramic material, the flexible circuit including a non-conductive polymer film layer and a conductive lead layer, said polymer film layer having a first surface attached to said conductive lead layer and a second surface opposite to said first surface and said lead layer including at pattern of lead segments extending in a path across said film layer and protruding thereabove, a bonding layer of curable material bonding said flexible circuit to said active surface of said sheet such that said sheet of piezoceramic material is strengthened and protected by said bonding layer, and said bonding layer together with said flexible circuit forms a protective skin covering said active surface of said piezoceramic sheet, said lead segments of said lead layer protruding into said bonding layer and contacting said active surface of said piezoceramic material through microvoids created in said bonding layer under said lead segments to establish direct electrical contact with said active surface while being bonded thereto, and said curable material of said bonding layer filling between said lead segments of said lead layer to firmly bond the first side of said polymer film layer to said active surface of said sheet of piezoceramic material such that in-plane strain in said sheet of piezoceramic material is effectively shear coupled between said piezoceramic material and said second surface of said polymer film layer.
 2. A piezoceramic package according to claim 1, wherein the curable material is a highly cross-linkable polymer.
 3. A piezoceramic package according to claim 1, wherein the sheet of piezoceramic material is a plate having a thickness under approximately one millimeter.
 4. A piezoceramic package according to claim 3, wherein the piezoelectric plate has first and second cross dimensions, each cross dimension being greater than about one centimeter.
 5. A piezoceramic package according to claim 1, wherein the bonding layer is a planarizing layer of curable sheet material having a pattern complementary to the lead pattern.
 6. A piezoceramic package according to claim 3, wherein the piezoceramic sheet has a sheet plane, and flex circuits are bonded to the sheet in a pattern for applying an electric field in said sheet plane.
 7. A piezoceramic package according to claim 3, wherein the piezoceramic sheet has a sheet plane, and flex circuits contact the sheet to apply an electric field which varies in a direction normal to said surface plane.
 8. A piezoceramic package according to claim 1, wherein the lead layer has a comb pattern.
 9. A piezoceramic package according to claim 1, comprising two different piezoceramic sheets in two different respective recesses, which are actuated in different directions to produce torsional actuation of the package.
 10. A piezoceramic package according to claim 1, further comprising a circuit element within the package.
 11. A piezoceramic package according to claim 10, wherein the circuit element includes at least one of a shunt, a filter, an impedance matcher, a storage element, a power source, an amplifier, and a switch.
 12. A piezoceramic package according to claim 10, wherein the circuit element includes a controller.
 13. A piezoceramic package according to claim 1, wherein first and second piezoceramic sheets are located in different layers of the package and actuated for moving in different senses to bend the package.
 14. A piezoceramic package according to claim 1, constituting a device selected from among a vane, airfoil, shaker, stepper, stirrer, damper and sonicator.
 15. A piezo ceramic package according to claim 1, wherein thickness of the package is less than twice a combined thickness of piezoceramic plates stacked in the package.
 16. A piezoceramic package according to claim 1, wherein the package forms a mechanical article selected from among a stack, flexure, shell, plate and bender.
 17. A piezoceramic package according to claim 1, configured as one of a pusher, flap, lever, bender, bellows and combination thereof. 